This might be intended as a reference that does nothing when clicked and might be accompanied by setting onclick etc. to actually do anything.

upd: While some say that putting a script in href is the same as putting it in onclick, there are some differences, like what happens on right click (and some scripts definitely should not be opened in a new tab or such). Still, my opinion of the practice is that it is somewhat ugly, and not only because of uninformative text in status bar when mouse over the link.

You can imagine that as someone copying that "javascript:;" into the url box and hitting Enter. Since it starts with "javascript:" the browser will know to execute what follows as javascript in the current page. Since all that follows is a ";", nothing will happen, actually. That's the same as clicking a button that has a onClick attribute set to ";".

For example:

<a href="javascript:alert('howdy!')">Click me!</a>

has the same effect that

<button onClick="alert('howdy!')">Click me!</button>

or even

<a href="#" onClick="alert('howdy!')">Click me!</a>

Keep in mind that, since it's a link, most browsers will render its address (in that case "javascript:alert('howdy!')") in the status bar, and that may be undesired (I find it to be particularly ugly).